This invention relates generally to clamps suitable for gripping structural members such as metal plates, beams, angles and the like in order, for example, to lift such a member and to hold and manipulate it for performing industrial operations thereon, or for assembly into a structure.
The prior art contains numerous types of clamps suitable for the gripping of metal plates and the like. Typically, such clamps incorporate one generally fixed jaw and another movable jaw which may be brought into contact with the structural member to be lifted. Certain prior art clamps, known as screw clamps, have incorporated a threaded shaft for advancing one jaw into engagement with the structural member. Commonly assigned U.S. Pat. Nos. 4,183,571 and 4,850,630 disclose screw-type clamps which provided significant improvements over previously existing clamp designs through the use of particular pivotable jaws (material gripping elements). These clamps provide a superior gripping of a clamped structural member, and at the same time eliminate the need to apply an extreme compressive force to the member via the screw. With these clamps, a portion of at least one of the material gripping surfaces is pivotally urged against the surface of the structural member by virtue of lifting torques developed about a pivot axis of a gripping element upon lifting a structural member secured within the clamp. As a result, the gripping element bites into the structural member to provide a secure grip during lifting.
The clamps as described above utilize various means for attachment to a lifting device. In one arrangement disclosed in above-mentioned U.S. Pat. No. 4,188,571, a lifting shackle is attached to the clamp body for pivotal motion in the general plane of the clamp body about a pin extending in a direction perpendicular to the general plane of the clamp body. In a second disclosed embodiment, the clamp incorporates a shackle additionally having a second pivot axis perpendicular to the first pivot axis. The pivotable shackles of these clamps allow, e.g., a steel plate gripped by the clamp at one edge to be rotated from an initial horizontal position through 180.degree.. Such manipulation is useful for handling plates at rolling and forming machines, for example. The provision in the second embodiment of a second pivot axis perpendicular to the first pivot axis allows the clamp to be side-loaded up to 90.degree..
The clamp of above-mentioned U.S. Pat. No. 4,850,630 incorporates directly in the clamp body at least one aperture for connection of a separate shackle. This design is intended principally for lifting applications not requiring substantial rotation of the lifted member, although, depending on the shackle arrangement employed, a limited degree of material rotation can be obtained.
The above-described clamps advantageously allow a structural member to be rotatably manipulated, at least to a degree, during lifting. However, when a pair of these clamps are positioned at opposite edge portions of a structural member such as a plate to perform a horizontal lift, the clamps do not allow for significant rotation of the structural member about a substantially horizontal axis in order to, for example, facilitate various operations such as welding to be performed on each of opposite surfaces of the member. Furthermore, these clamps, used alone or in pairs, do not allow a full 360.degree. rotation of a lifted member relative to the lifting shackle about any axis.
Commonly assigned U.S. Pat. No. 3,342,521 discloses a lifting clamp having a shackle pin which is initially slideably and rotatably secured to a shackle block in alignment with a material-receiving slot of the clamp. Upon application of a lifting force to the shackle pin during a lifting operation, the shackle block pivots together with the pin out of alignment with the material-receiving slot so as to create a torque gripping action on the article being lifted by the jaws. The shackle pin is not intended to rotate freely during a lifting operation and has no provision to allow the same. Furthermore, to the extent the pin may be somewhat rotatable during lifting, since the shackle pin is, during lifting, translationally and angularly offset from its original position in alignment with the material-receiving slot, the clamp would not be functional, in conjunction with another like clamp, to allow rotation of a structural member about a substantially horizontal axis during a horizontal lift.
Commonly assigned Canadian Patent No. 511,259, discloses a screw-type clamp wherein a cylindrical portion of the screw has mounted thereon a swivel. The swivel is adapted to be rotatably held on the screw by thrust-bearing surfaces. While rotation of a lifted work piece is possible with this clamp, since the swivel axis is coincident with the screw and substantially offset from the material-receiving slot, torques will be developed on the swivel during lifting which will tend to bind the swivel and the thrust bearing surfaces and thereby inhibit free rotation of the clamp during lifting. Such torques may also have the adverse effect of bending the screw to which the swivel is mounted. Furthermore, due to the location and orientation of the swivel, the clamp would not be usable in conjunction with another like clamp to allow rotation of a structural member such as a plate about a horizontal axis during a horizontal lift.
It is known to provide lifting hooks and tackle with 360.degree. rotatable swivels. Typically in such devices a rotatable swivel is provided directly above a hook or other connecting means and has attached thereto a shackle arrangement whereby a lifted work piece may be rotated through 360.degree. about a vertical axis. U.S. Pat. Nos. 381,043; 2,708,999; 2,625,005 and 2,925,298 are exemplary of such known devices. These devices are not suitable for directly clamping and lifting a structural member, nor for allowing rotation of a lifted member about a substantially horizontal axis.
There thus exists a need for a clamp design which is capable of positively and securely gripping a structural member for various lifting and manipulating operations, which affords a rotational degree of freedom allowing the structural member to be freely rotated through 360.degree. relative to a lifting shackle, and which allows, through the use of a pair of clamps, free rotation of a lifted structural member about a horizontal axis.